Skip running preview-website workflow in forks with credentials

[jsoref]

The https://github.com/voyagermesh/voyager/blob/master/.github/workflows/preview-website.yml workflow absolutely requires credentials that forks will not have:

voyager/.github/workflows/preview-website.yml

Lines 45 to 53 in 6536e30

	- name: Clone website repository
	env:
	GITHUB_USER: 1gtm
	GITHUB_TOKEN: ${{ secrets.LGTM_GITHUB_TOKEN }}
	WEBSITE_REPOSITORY: ${{ secrets.WEBSITE_REPOSITORY }}
	run: |
	url="https://${GITHUB_USER}:${GITHUB_TOKEN}@${WEBSITE_REPOSITORY}.git"
	cd $RUNNER_WORKSPACE
	git clone $url

If you're interested in letting it run in forks, then the first step should check for the presence of the required secrets, and if the they're missing, stop. This can be done with a step that sets an output, either setting an output for a job and then having the core job need that job and condition itself on the output, or by having each subsequent step if to the output.

I can provide a PR to implement either of the above.